The present invention relates to by-products that are produced in flue gas desulfurization (FGD) processes using alkaline earth metal compounds, e.g. calcium hydroxide, as absorbent. Such products usually contain substantial amounts of alkaline earth metal sulfites. The sulfite contents make the products less suitable for disposal in some locations and also impede possible industrial use for the products.
More specifically the present invention relates to a process of converting sulfite, e.g. calcium sulfite, in FGD by-products into calcium sulfate thereby making the by-products more acceptable for disposal to the environment and more suitable for industrial use e.g. for mixing with cement.
Most FGD processes utilize a basic absorbent and may be either regenerative where the spent absorbent is regenerated for reuse in the process or non-regenerative processes where the spent absorbent is disposed of as land fill, or utilized industrially for other purposes.
In the latter type of FGD processes alkaline earth metal compounds, particularly calcium hydroxyde and calcium carbonate, have found wide use as absorbents, and whether the FGD process is carried out by wet scrubbing, spray dryer absorption or cleaning with dry particulate absorbent, the spent absorbent will contain substantial amounts of sulfite. Dependant on the FGD process used and the extent to which fly ash present has been separated from the flue gas prior to the FGD process the spent absorbent may further contain various amounts of fly ash.
The sulfite may be converted into sulfate by various oxidation processes. When the spent absorbent is a dry particulate material, oxidation may be carried out in the dry state by bringing the material in contact, e.g. in a fluidized bed, with a hot oxygen-containing gas. The process of this type is described in European patent application No. 74 258. Operation of a fluidized bed apparatus for this process is, however, difficult because the particle size of this type of material is usually too small, typically 10-100 microns, to from a fluidized layer, unless the fluidization gas velocity is so low that the process is uneconomical.
It might be possible to enlarge the particle size by agglomeration or pelletizing prior to the fluidized bed treatment. Such particle size enlargement processes are conventionally made by admixing of suitable binders in connection with mechanical treatment, e.g. on a rotating disc or by compacting.
This would be a possible solution to the problem, and an increased particle size may also be desirable with respect to handling and various applications of the material. Such further processing steps, however, add to the total processing costs, and it is therefore also an object of the present invention to obtain an agglomerated product without adding further processing steps. In other words, in the present invention the process of converting sulfite into sulfate and agglomeration of the material are made simultaneously.
The material which may be treated according to the invention have to a certain extent glassy properties and as will be explained later, the present invention makes use inter alia of these properties, which imply that the particles become sticky at elevated temperatures and may thereby form agglomerates. The particles will begin becoming sticky upon heating above a temperature which in the present specification and claims is termed the agglomeration temperature of the material.